Fires that rage in thousands of underground coal seams around the world are polluting the air and releasing millions of tons of carbon dioxide, the major greenhouse gas. Although coal fires occur naturally from spontaneous combustion, scientists say the frequency of such fires has risen as mining has exposed coal deposits to more fires and to the oxygen that feeds them. The Office of Surface Mining says coal fires have cost the country about $1 billion to date — and that’s a drop in the bucket compared to the international cost. In China, for example, coal fires burn upward of 200 million tons of coal per year, about 20 percent of national production. Those fires alone produce almost as much CO2 as all the cars and small trucks in the U.S.